Lots to see in Colorado, and you'll be right in the foothills of the Rocky Mountains. Checkout the National Parks Web site to get an idea of what's in the area as far as NPS campgrounds: www.nps.gov
The first parks that come to mind are Rocky Mountain National Park near Estes Park and the Pike's Peak, down near Colorado Springs. Both are great destinations in the warmer months. (Also great in the winter, but cold, cold, cold!)
As far as campgrounds, the Denver/Boulder area seems to have very few in town, although there may be some residential trailer parks that have transient sites (usually not the best). One place that is beautiful, but not very close is Chatfield Reservoir/State Park. It's just below the foothills in southwest Denver and probably 30-45 minutes away from Boulder (perhaps longer in rush hour). Spaces are spread out with lots of open areas, with grass and a some trees, picnic tables, BBQs and firepits, with full hookups. There are also recently renovated bathrooms, showers and laundry facilities. It might not be real close to your wedding, but it's pretty close to most of Denver, and conveniently located near the freeways (but far enough away to avoid any noise). I think it is in the city limits, but it feels like you are farther out of town.
As for other things to see, just head up into the mountains and you'll never leave Colorado...
Also, if you venture into the national parks, get the annual pass. It's $80, but you'll break even after a couple of entrance fees; and it's good for the next 12 months! If you're 62, the senior pass is $10; and it's a lifetime pass, with 1/2 off on most NP campgrounds. More info on the NPS site.